Curl-resistant transfer elements and methods for preparing same



United States Patent 3,453,135 CURL-RESISTANT TRANSFER ELEMENTS AND METHODS FOR PREPARING SAME Albert E. Brown, Glen Cove, N.Y., assignor to Columbia Ribbon and Carbon Manufacturing Co., Inc., Glen Cove, N.Y., a corporation of New York No Drawing. Filed Oct. 10, 1966, Ser. No. 585,285

Int. Cl. B41c US. Cl. 117-36.4 8 Claims ABSTRACT OF THE DISCLOSURE Paper has the tendency to swell and change its dimensions under the effects of changes in atmospheric conditions, particularly humidity. This is due to the fact that paper is composed of cellulose fibers which are generally arranged parallel to the direction of the paper as it travels through the paper-making machine. Some papers have a random distribution of cellulose fibers, however. Regardless of distribution, cellulose fibers are hygroscopic and swell when the humidity is increased or the temperature is lowered. Conversely they shrink when the humidity is reduced or the temperature is increased. The amount of swelling or shrinkage is dependent upon the cellulose content of the paper and the presence of additives such as wet-strength agents.

In general most papers will expand perpendicular to the direction of the cellulose fibers to an extent ranging from 2% to 5% or more of their normal size when they absorb more than normal moisture from the atmosphere, such as when storedin a damp place or when exposed on a humid or rainy day. The shrinkage is about equal in degree when paper is stored in a hot dry place or ex posed to sunlight or to a hot dry atmosphere.

The problems caused by the dimensional changes in paper are varied but one of the most common problems is curling or rolling of the paper. This is most pronounced in cases where one surface of the paper has been coated or bonded to another flexible sheet. When the paper is in this state and its moisture content is changed, the paper tends to expand or shrink 'but the coating or sheet to which it is 'bonded tends to retain its dimension. This causes the coated or bonded paper to curl or roll towards the treated surface when the moisture content is increased and away from the treated surface when the moisture content is reduced.

One of the most important problems encountered in the carbon paper field, particularly with the use of transfer layers based upon resinous binder materials, is the tendency of the transfer element to curl or roll or lose its flatness, particularly with substantial changes in temperature and/or humidity.

The seriousness of the problem varies depending upon the type of paper used, the nature of the transfer coating and the end use to which the transfer element is to be applied. Greatest seriousness is encountered with the production of transfer sheets on which the transfer layer is of the squeeze-out type based upon a resinous binder material and applied by means of a volatile solvent, due primarily to the resinous nature of the binder, its open porous structure and the presence of varying amounts of residual volatile solvent. US. Patents Nos. 2,810,661; 2,-

3,453,135 Patented July 1, 1969 984,582 and 2,944,037 are illustrative of such transfer sheets.

There have been many attempts to overcome the curling problem of paper but there has been no recognized solution up to the present. Most proposed solutions are attempts to counteract the curling tendency of the paper rather than attempts to overcome the cause of the curl. According to these proposals, the paper foundation is provided with a waxy or resinous coating on the surface opposite to the treated surface, such as one carrying a transfer layer, so that the paper foundation is fairly uniformly coated on both sides and theoretically will not curl in either direction. However, such back coatings are not at all effective in many cases, particularly in the field of solvent carbons based upon resinous binder materials and containing pressure-exudable oil-and-pigment ink, due to the necessary differences in composition and structure between the back coating and the ink-releasing layer.

Such back coatings are also undesirable in the carbon paper field in that they increase the thickness of the transfer element, cushion the impact of the type bar and otherwise reduce the quality of the copy produced,

It is the primary object of the present invention to produce curl-resistant dimensionally-stable transfer elements based upon paper by avoiding the cause of the curl in the paper itself and thereby providing a curl-resistant transfer element of varied utility such as for use as carbon papers and ribbons of all types, and most particularly for solvent carbons of the squeeze-out type.

It is another object of this invention to eliminate the necessity of applying coatings of wax or resin to the rear surface of transfer elements, which coatings interfere with the imaging properties of the transfer elements.

Other features and advantages will hereinafter appear.

The present invention is based upon the discovery that the cause of curl-formation in transfer elements based upon paper, namely the absorption and loss of moisture in the paper due to changes in temperature and/ or humidity, can be overcome by treating the paper with a liquid methoxy polyethylene glycol prior to the application of the transfer layer.

It is not at all clear how or why this particular class of materials renders paper dimensionally stable and moisture-resistant while other materials which are so similar in physical and chemical properties do not have this ability. However, it has been found that the methoxy polyethylene glycols are rapidly and completely absorbed by the cellulose fibers of the paper and are strongly bonded thereto, possibly by means of hydrogen bonding, so that they cannot be displaced by water or moisture. This feature appears to be the key to the present invention.

The methoxy polyethylene glycol is preferably applied to the paper as a dilute solution containing from about 5% to 50% by weight of the glycol dissolved in a volatile solvent which is preferably water but which may also be an organic solvent such as methyl ethyl ketone, ethyl acetate, methanol, ethanol or similar convenient volatile organic solvent for the glycol. However, a glycol such as Carbowax 350 may be applied in undiluted condition since it is a relatively thin liquid, provided that it is permitted to remain on the paper for a long enough period of time to be completely absorbed, generally several weeks, before the paper is coated with the transfer composition and/or other coatings.

Water is the preferred solvent since it permits almost an instantaneous absorption of the methoxy glycol by the paper, apparently because the water has an affinity for the paper fibers and causes them to swell and take in the methoxy glycol. The expansion of the paper is almost instantaneous whereas when organic solvents are used instead of Water, the expansion of the paper and absorpice tion' of the methoxy glycol may require the passage of from one to two weeks to insure completion and stabilization before other coatings can be applied. In cases where the paper does not have good wet strength properties, it is preferred to use a solvent mixture consisting of water and a water-miscible organic solvent such as methanol. The water preferably comprises to 25% by weight of the mixture in the case of normal untreated papers but may be varied depending on the wet strength of the particular paper being treated.

The stabilizing composition may consist essentially of the methoxy polyethylene glycol compound although other additives may be included provided that they do not interfere with the destruction of the curling tendency of the paper. For instance, small amounts of dissolved dyestuif, pigment, opacifying filler, anti-static compound, or the like, may be added to alter the color, opacity, static properties and/or feel of the paper without interfering with the curl properties of the paper. The amount of glycol applied varies a great deal depending upon the nature of the paper itself. Some papers are very soft and absorbent due to their open porous structure, while others are more heavily sized and calendered and thus are harder, more compact and less absorbent. In general the glycol'is applied in an amount equalling from about 2% up to about 35% by weight based upon the weight of the paper, which equals from about 0.33 gram up to about 5.9 grams per square meter.

The papers which are commonly used as transfer sheet and ribbon foundations and show the greatest tendency to curl are the tissue type papers having a weight of from about 2 to about 10 pounds per ream. However, the present invention applies equally well to cellulose fiber papers of all types and weights and for all uses where curling or rolling of the paper presents a problem. A ream, as used herein, is equal to 480 sheets which are 20" x 30 in dimension.

The following example is given by way of illustration and should not be considered limitative.

A wide web of Schweitzer 6 pound per ream tissue stock is treated with methoxy polyethylene glycol by passing it against an application roller carrying a supply of the glycol. The glycol is supplied to th application roller in the form of a 20% solution of Carbowax 350 in a mixture of 20% Water and 80% methyl alcohol. The web is passed over the roller at a speed of about 60 feet per minute so that the amount of glycol picked up by the paper is about 0.77 pound per ream which equals about 1.88 grams/square meter or about 11.2% based upon the weight of the paper. The solvent mixture is evaporated from the paper and the glycol is completely absorbed so that the paper is dry to the touch and nearly completely identical in appearance to the untreated paper. Howver, the treated paper feels limp and less brittle than the untreated paper. The glycol causes the cellulose fibers in the paper to swell somewhat so' that the paper web is about 1.5% wider and slightly thicker than the untreated paper. However, the treated paper is now dimensionally stable and will not shrink or expand measurably under the elfects of changes in humidity and is substantially completely resistant to curling or rolling when the transfer layer is applied.

It is preferred in many cases to apply an undercoating to the paper foundation prior to the application of the transfer layer. The undercoating may be a wax layer or a resinous layer and may function as either a release layer, to assist the complete transfer of the transferable layer, or as a bonding layer, to prevent the transfer layer from peeling or flaking from the foundation or from transferring from the foundation under the effects of imaging pressure. These differences in function depend upon the types of materials used and the results desired. US. Patents Nos. 3,037,879; 3,049,457 and 3,170,809 are illustrative of such intermediate layers, their compositions and their functions.

Similarly, in many cases it is desirable to apply a back coating to the opposite surface of the paper foundation for many purposes other than to prevent curling, such as anti-slip coatings, anti-static coatings or coatings which improve the appearance and/or printability of the rear surface of the transfer element.

In all cases it is important that the coatings which are in direct contact with the paper foundation must be chemically incompatible with or non-solvents for the methoxy polyethylene glycolin the paper foundation. Otherwise the glycol may be leached out of the paper into the coating with which it is compatible or soluble and the curl-resistance of the paper will be reduced. Since the glycol is incompatible with most oleophilic waxes and resins, there is very little problem involved in avoiding the use of compatible binder materials for the back coating, intermediate layer and/or transfer layer.

The following example is illustrative of one suitable intermediate layer composition and one suitable transfer layer composition and the method of applying these compositions to the glycol-treated papr foundation. In preparing transfer elements of the squeeze-out type, it is desirable to use a binding layer as the intermediate layer to retain the resinous structure of the transfer layer on the paper foundation while the oil-and-pigment ink is exuded under the elfects of imaging pressure.

A suitable binding layer of this type is formed on the foundation by applying the following composition to the paper foundation in a weight of about one pound per ream and solidifying the layer by evaporating the solvent.

Ingredients: Parts by weight Vinyl chloride-vinyl acetate copolymer (Vinylite VYHH) 15 Ethyl acetate A pressure-sensitive ink-releasing layer is then applied over the binding layer by coating the layer with the following composition, applied in a weight of about six pounds per ream. The ink-releasing layer is hardened by evaporation of the volatile solvents.

Ingredients: Parts by weight Vinyl chloride-vinyl acetate copolymer 10.0 Mineral oil 27.5 Toned carbon black 7.5 Toluol 15.0 Ethyl acetate 45.0

The dimensional stability of the present papers is considered to be completely unobvious in view of the fact that the methoxy polyethylene glycols are the only compounds, of the large group tested, which produce this result. Other closely-related water-soluble liquids and solids were found completely unsuitable, such as ethylene glycol, glycerine, stearic acid, cetyl alcohol, the straight polyethylene glycols (Carbowaxes), and other related compounds. Most of these materials have no effect on the lack of stability of the paper while others improve the stability to an insignificant extent and are compatible with most waxes and/or resins so as to be unsuitable for use where the paper is to be coated with such materials.

Variations and modifications may be made within the scope of the claims and portions of the improvements may be used without others.

I claim:

1. The process for producing a pressure-sensitive transfer element which is resistant to curling under the effects of variations in humidity which comprises uniformly treating a cellulosic paper foundation sheet with a liquid composition comprising from about 2% to about 35% by weight, based upon the weight of the paper, of a methoxy polyethylene glycol to cause the paper fibers to swell, and then applying to said foundation a pressure-sensitive trans fer layer comprising pressure-transferable imaging material.

2. The process of claim 1 in which the glycol is applied as a solution dissolved in a volatile solvent and the solvent is evaporated.

3. The process of claim 1 in which the paper foundation is uniformly impregnated with the glycol.

4. The process of claim 1 in which the glycol is applied as an aqueous solution containing from about 5% to 50% by weight of the glycol, and the water is evaporated.

5. The process of claim 1 in which an undercoating is applied between the treated paper and the transfer layer, the glycol being chemically incompatible with the composition of the undercoating.

6. A pressure-sensitive transfer element which is resistant to curling under the effects of variations in humidity which comprises a cellulosic paper foundation having uniformly absorbed in the swollen fibers thereof a liquid composition comprising from about 2% to about 35% by weight, based upon the weight of the paper, of a methoxy polyethylene glycol, and having thereon a pressure-sensitive transfer layer comprising pressure-transferable imaging material.

7. A paper according to claim 6 in which the glycol is present as a uniform impregnation throughout the paper sheet.

8. A transfer element according to claim 6 in which an undercoating is present between the paper and the transfer layer, the glycol being chemically incompatible with the composition of the undercoating.

References Cited MURRAY KATZ, Primary Examiner.

US. Cl. X.R. 

